With their versatility and compact nature, semiconductor devices are now used in virtually all electronic equipment. Due to their complex structure and small size, semiconductor devices may be extremely fragile. The devices are, therefore, typically packaged to protect them from abrasion or shock which could easily destroy them. Along with protecting the device itself, semiconductor packaging should also be as compact as practical, while facilitating electrical contact to the device within the package. This enables electrical components containing semiconductors to be commensurately smaller.
Surface mount technology is often employed to keep the package for an integrated circuit as compact as possible. Surface mount technology reduces the size of a package by eliminating certain parts, such as sockets, which are unnecessary for the semiconductor's operation. One type of surface mount package is the ball grid array package. A ball grid array package not only provides a substrate on which the semiconductor device is mounted, but also the electrical contact between the bonding pads of the semiconductor device and a circuit board. The ball grid array package mounts onto a circuit board.
In a ball grid array package, the bottom surface of the semiconductor device is attached to the top surface of the package substrate. Electrical connections are made between the bonding pads on the top surface of the device and contacts on the substrate. After these electrical connections are made, the semiconductor device is encapsulated in some manner to protect it. Depending on the specific semiconductor device or the environment for its use, the device may be encapsulated using, for example, epoxy or a hermetic seal.
The ball grid array package is named for the solder balls on the bottom surface of the substrate, opposite the semiconductor device. Disposed in a grid array, the solder balls are used to make contact with the circuit board. Inside the package are conductive traces which electrically connect the solder balls to the contacts on the top surface of the package substrate where the semiconductor device is attached. After placing the package with its solder balls over the contact areas of a circuit board, the package is attached to the circuit board by heating and melting the solder balls.
Bonding thin gold or gold alloy wire from the semiconductor's bonding pads to a corresponding contact on the ball grid array package substrate provides electrical connections between the semiconductor device. This conventional wire bonding process has several shortcomings. First, the wire bonding can cause excessive inductance to the electrical signal. Second, wire bonding requires bonding pads at the periphery of the semiconductor device. This can limit the amount of bonding pads on the die, which corresponds to the number of input and output signals available. Third, with peripheral bonding pads, power cannot be directly supplied to the interior of the semiconductor device.
Another method of making electrical connection between a semiconductor device and a semiconductor package does not require any bonding wires. This method employs a specifically designed integrated circuit known as a flip-chip. Flip-chips do not require bonding wires to form electrical connections. A flip-chip has bonding pads spaced around the entire top surface of the semiconductor device, rather than predominantly at the chip's periphery, as is common with a more traditional device requiring wire bonding. On each bonding pad of the flip-chip is placed a small solder bump. Instead of mounting the back of the flip-chip to the substrate, the flip-chip is flipped over so that the solder bumps make direct contact with appropriately arranged contacts on the package substrate.
By eliminating bonding wires, a package designed for a flip-chip greatly reduces inductance between the semiconductor device and the substrate, and allows the electrical signal to be supplied directly between the bonding pads on the semiconductor device and the contacts and traces on the substrate. Additionally, since flip-chips have bonding pads spaced across the entire top surface of the semiconductor device, rather than predominantly at the periphery of the device, a flip-chip can have far more bonding pads than a wire bonded chip. Also, with the bonding pads located across the interior of the flip-chip, power can be supplied directly to any location on the device.
Package substrates are usually made of a multi-layered material, such as printed circuit board, or flex circuit. A substrate may contain a portion of the required traces on each of several layers of the substrate. Conductive through-holes form the electrical connections between the traces on the different layers of the substrate. Because many electrical connections need to be made on the substrate directly under the flip-chip, that area contains many through-holes with a very fine pitch between them. With current technology, a through-hole required between any two layers of the substrate must be formed in the same position in all of the layers. That is, the through-hole forms a conductive path from the top surface of the multi-layered substrate, where the flip-chip is attached, to the bottom surface of the substrate.
To form a ball grid array package, the solder balls are attached to the bottom surface of the package substrate and connect with the appropriate through-holes. Unfortunately, the through-holes required to make electrical connection to a flip-chip will typically have a diameter and a pitch that is far smaller than the diameter of the conductive pads on which the solder balls are formed. A single solder ball conductive pad could, therefore, contact multiple through-holes creating unintended, and undesired, shorts and loops in the resulting circuit.
To circumvent this problem, traditional technology places all of the solder balls in an area of the package substrate outside the area occupied by the flip-chip. Thus the solder balls reside in an area of the package substrate having only the necessary through-holes to contact the solder balls and not in the area under the flip-chip which is densely populated with through-holes. Unfortunately, this technique tends to increase the package size, moving all of the solder balls to the periphery of the package and not utilizing the center of the package where the flip-chip is attached.
A need exists, therefore, for a ball grid array package that can be efficiently used with a flip-chip. The ball grid array package should permit solder ball placement in the area of the substrate underlying the flip-chip. Adventageously, such an array may be smaller than conventional packages while having surface area sufficient for the number of solder balls required to make electrical contact between the semiconductor device and the package.